Modulating array transmitter autocalibration method and system

ABSTRACT

An autocalibrating modulating array transmitter  10  and autocalibration method  30 . The autocalibrating modulating array transmitter is used to gain-balance and phase-balance parallel amplifier stages  11  for coherent combining. The novelty of the autocalibration technique involves periodic substitution of calibration symbols into a valid data stream, the use of only the error magnitude (as opposed to magnitude and phase), and a random selection of either gain or phase adjustments to avoid limit cycles.

BACKGROUND

The present invention relates generally to modulating arraytransmitters, and more particularly, to an autocalibrating modulatingarray QAM transmitter system and method.

Heretofore, the assignee of the present invention has developedmodulating array transmitters. For example, U.S. Pat. No. 5,612,651,assigned to the assignee of the present invention, discloses a“Modulating Array QAM Transmitter”, and U.S. Pat. No. 5,381,449 issuedto Jasper et al. disclose “Peak to Average Power Ratio ReductionMethodology for QAM Communications Systems”, for example. However todate, there has not been any system or method that provides forautocalibration of such modulating array transmitters.

It is therefore an objective of the present invention to provide forautocalibrating modulating array QAM transmitter systems and methods.

SUMMARY OF THE INVENTION

To meet the above and other objectives, the present invention comprisesautocalibrating modulating array QAM transmitter systems and methods.The present invention provides for self alignment of elements of amodulating array transmitter to maintain high waveform precision. Thepresent invention is self correcting, requiring no manual alignment orrealignment of transmitter elements, which is particularly important insatellite transmitters, for example. The present invention alsomaintains high precision of the communications waveform over thelifetime of the transmitter so that overall communications efficiency ismaintained.

An exemplary autocalibrating modulating array transmitter systemcomprises a plurality of discrete, parallel quadrature power elementsthat each include a QPSK modulator coupled by way of a gain/phaseadjustment circuit to a high-power amplifier driven to saturation. Databits are input to a bit-to-symbol mapper whose outputs are coupled tothe respective quadrature power elements. Outputs of the quadraphasepower elements are combined in a passive network to produce amultilevel, high-power RF output signal. The output signal is sampled bya coupler and input to an autocalibration controller having outputscoupled to the bit-to-symbol mapper and to one of the gain/phaseadjustment circuits. The autocalibration controller comprises a signleanalog-to-digital converter that is used to sample the RF output signal.

An exemplary method or algorithm that is implemented in theautocalibrating modulating array transmitter system, and in particularin the autocalibration controller, comprises the following steps. A pairof quadraphase power elements is selected, one of which is adjusted, theother of which is held constant. A random decision is made to dithereither the gain or the phase of the selected quadraphase power element.If the previous gain (phase) dither increases the error magnitude, thesign of the dither is reversed. The gain (phase) of the selectedquadraphase power element is adjusted by the small dither amount(positive or negative).

A calibration symbol is sent, wherein the two phase-opposed outputsignals of the quadraphase power elements are designed to cancel eachother. An analog-to-digital converter measures the error magnitude.Then, a predetermined number (10⁵) of data symbols are sent, and theprocessing steps are repeated starting at the decision step, until theerror magnitude is reduced below a threshold value. A new quadraphasepower element is selected, and the above steps are then repeated, usingthe previously selected quadraphase power element as a reference,starting from the second step.

BRIEF DESCRIPTION OF THE DRAWINGS

The various features and advantages of the present invention may be morereadily understood with reference to the following detailed descriptiontaken in conjunction with the accompanying drawings, wherein likereference numerals designate like structural elements, and in which:

FIG. 1 is a block diagram of a three-stage modulating array transmitterautocalibration system implemented in accordance with the principles ofthe present invention;

FIG. 2 illustrates an error signal representing one calibration symbolthat is the vector sum of two-component quadraphase power elementoutputs:

FIG. 3 illustrates an initial distorted 128QAM constellation, wherein“X”s are the transmitted points, and “+”s are the desired points, andwherein transmitter noise is not included:

FIG. 4 illustrates a 128 QAM constellation after 50 iterations:

FIG. 5 illustrates a final converged constellation after 100 iterations,and 1100 quadraphase power element updates:

FIG. 6 illustrates constellation point trajectories as theautocalibration system converges: and

FIG. 7 is a flow diagram illustrating an exemplary modulating arraytransmitter autocalibration method implemented in accordance with theprinciples of the present invention.

DETAILED DESCRIPTION

Referring to the drawing figures, FIG. 1 is a block diagram of athree-stage modulating array transmitter 10 comprising anautocalibration system 20 implementing in accordance with the principlesof the present invention. In general, the modulating array transmitter10 has an architecture that provides for digital radio communications.

The modulating array transmitter autocalibration system 20 is used togain-balance and phase-balance parallel amplifier stages 11 for coherentcombining. The novelty of the autocalibration system 20 lies in theperiodic substitution of calibration symbols into a valid data stream,the use of only the error magnitude (as opposed to magnitude and phase),and a random selection of either gain or phase adjustments to avoidlimit cycles.

The basic design and operating principles of the modulating arraytransmitter 10 is described U.S. Pat. No. 5,612,651, assigned to theassignee of the present invention, the contents of which areincorporated herein by reference in their entirety. In general, themodulating array transmitter 10 converts digital data into a radiofrequency signal for spectrum-efficient, over-the-air transmission.

In summary, the modulating array transmitter 10 is a high-power,direct-conversion QAM (quadrature amplitude modulation) modulator. Themodulating array transmitter 10 is comprised of a plurality of discrete,parallel stages 11, referred to as quadrature power elements (QPE) 11.Each quadraphase power element 11 comprises a QPSK modulator 12 and a(solid-state) high-power amplifier 13 driven to saturation. The QPSKmodulator 12 is coupled to the high-power amplifier 13 by way of again/phase adjustment circuit 14.

Data bits input to the modulating array transmitter 10 are supplied to abit-to-symbol mapper 15 whose outputs are input to the plurality ofquadrature power elements 11. Each quadrature power element 11 has anon/off keying input for receiving an on/off keying bit input signal thatselectively keys the modulator on and off, and a local oscillator input(LO) for receiving a reference input signal.

The quadrature power element 11 modulates the reference input signal inaccordance with the digital input signals and outputs a modulated RFexcitation signal which is subsequently amplified by the high-poweramplifier 13. Outputs of the quadraphase power elements 11 are combinedin a passive network 16 or power combiner 16 to build a multilevel,high-power RF output signal.

The high-power RF output signal is sampled by a coupler 17 and input toan autocalibration controller 30 in accordance with the presentinvention. The autocalibration controller 30 has outputs that arecoupled to the bit-to-symbol mapper 15 and to one of the gain/phaseadjustment circuits 14. The autocalibration controller 30 comprises asingle analog-to-digital converter (ADC) 31 that is used to sample thehigh-power RF output signal.

To form a desired high-order constellation of transmitted symbols, suchas 64QAM, 128QAM, 256QAM, and so forth, the quadraphase power elements11 must be in careful gain and phase alignment. Since these gains andphases may drift over the long-term life of the transmitter 10, adaptivecorrection is required.

Details of the autocalibration system 20 will now be described. Theapproach used in implementing the autocalibration system 20 relies onforward error-correction (FEC) in the communications link. Since mostpractical digital communications systems employ error-correction coding,this is not an onerous requirement. The concept of operation of theautocalibration system 20 is that once every 10⁵-10⁶ symbols, acalibration symbol is substituted for a valid QAM data symbol. Sincethis substitution rate occurs well below the typical FEC threshold forerror-correction, these symbols are automatically corrected at areceiver, and no data is lost. Each calibration symbol is ideally a zerosignal, generated by the sum of two phase opposed outputs of aquadraphase power element 11. The actual symbol observed is therefore anerror signal for the two selected quadraphase power elements 11.

The analog-to-digital converter (ADC) 31 of the autocalibrationcontroller 30 is coupled to the high-power output of the modulatingarray transmitter 10 and samples the magnitude of the calibrationsymbol. Phase is not measured, because this would require a coherentreceiver or quadrature down-converter to be incorporated in thetransmitter 10. Since only the error magnitude is available, the outputsamples are used to alternately dither the gain and phase of onequadraphase power element 11 over subsequent calibration symbols untilthe error is driven to zero. This process is repeated for allquadraphase power elements 11 in a round-robin fashion. If a “hot” sparequadraphase power element 11 is included in the modulating arraytransmitter 10, this approach ensures that the hot spare quadraphasepower element 11 is always ready to be switched-in.

Referring again to FIG. 1, it is a block diagram of a three-stagemodulating array transmitter 10, capable of modulating 16QAM. Theautocalibration system 20 comprises the autocalibration controller 30that adjusts the gain and phase of the input to the high power amplifier13, as well as the symbol mapping and scheduling of calibration symbols.

The autocalibration system 20 employs a autocalibration algorithm 30 ormethod 30 that will now be described. FIG. 2 depicts how the calibrationsymbol generates an error in signal space. The autocalibration algorithm30 or method 30 is straightforward, and is summarized below. Theautocalibration algorithm 30 or method 30 is pictorially illustrated inthe form of a flow diagram shown in FIG. 7.

A pair of quadraphase power elements 11 is selected 31, one of which isadjusted, the other of which is held constant. A random decision 32 ismade to dither either the gain or the phase of the selected quadraphasepower element 11. If the previous gain (phase) dither increases theerror magnitude, the sign of the dither is reversed 33. The gain (phaseof the selected quadraphase power element 11 is adjusted 34 by the smalldither amount (positive or negative).

A calibration symbol is sent 35, wherein the two phase-opposed outputsignals of the quadraphase power elements 11 are designed to cancel eachother. The analog-to-digital converter (ADC) 31 measures 36 the errormagnitude. Then, 10⁵ data symbols are sent 37, and the processing stepsare repeated 38 starting at the second step 32 (the decision step),until the error magnitude is reduced below a threshold value. A newquadraphase power element 11 is selected, and the above steps are thenrepeated 39, using the previously selected (target) quadraphase powerelement 11 as a reference, starting from the second step 32.

Essentially, each gain and phase adjustment is made as a one-dimensionalsteepest-descent algorithm. An exemplary one-dimensionalsteepest-descent algorithm is discussed in S. Haykin, “Adaptive FilterTheory,” 3rd ed. Prentice-Hall, N.J. 1996, for example. The simplegeometry of the problem (as is shown in FIG. 2) assures a single minimumfor each parameter. The random selection of gain dither versus phasedither is used to prevent limit cycles in the convergence of thealgorithm 30.

Simulation of the autocalibration procedure employed in theautocalibration system 20 has shown convergence to be relatively fast,with excellent results in less than 100 calibration symbols perquadraphase power element 11. Representative simulation results areshown in FIGS. 3-6. In this example, a 128QAM-generating modulatingarray transmitter 10 is considered, using eleven quadraphase powerelements 11. FIG. 3 shows a randomly chose, initial distorted signalconstellation due to gain and phase errors among the quadraphase powerelements 11. After fifty iterations on each quadraphase power element11, most of the distortion has been removed, as shown in FIG. 4. Onlythe most deviant quadraphase power elements 11 contribute to the error.After one-hundred iterations, the constellation in FIG. 5 is nearlyideal, and only very small errors remain.

Trajectories of the constellation points are shown in FIG. 6. A fewpoints in the constellation are shown, where each dot represents oneiteration of all eleven quadraphase power elements 11. The path to thedesired point is not direct, since this depends on the combination ofeleven different random errors. However, as the dots become more denselypacked (and convergence slows) the smaller errors are removed, and thetrajectory moves in the direction of the desired point.

Thus, autocalibrating modulating array transmitter systems and methodshave been disclosed. It is to be understood that the describedembodiments are merely illustrative of some of the many specificembodiments which represent applications of the principles of thepresent invention. Clearly, numerous and other arrangements can bereadily devised by those skilled in the art without departing from thescope of the invention.

What is claimed is:
 1. An autocalibrating modulated array transmittercomprising: a plurality of quadraphase power elements that eachcomprise: (a) a QPSK modulator having data inputs for receiving digitalinput signals, an on/off keying input for receiving an on/off keying bitinput signal that selectively keys the modulator on and off, and a localoscillator input for receiving a reference input signal, for modulatingthe reference input signal in accordance with the digital input signalsto output a modulated RF excitation signal; (b) a gain/phase adjustmentcircuit coupled to the QPSK modulator; and (c) a power amplifier coupledto the gain/phase adjustment circuit for receiving the modulated RFexcitation signal, for outputting an amplified quadrature amplitudemodulated output signal, and wherein selected pairs of power amplifiersthat are driven with opposing phases to form certain symbols are keyedoff by means of the respective on/off keying bit input signal coupled tothe respective QPSK modulator; a power combiner coupled to the pluralityof quadraphase power elements for combining the amplified quadratureamplitude modulated output signals derived therefrom to output aquadrature amplitude modulated output constellation containing aplurality of symbols; and an autocalibration controller having outputscoupled to the bit-to-symbol mapper and to one of the gain/phaseadjustment circuits and that comprises an analog-to-digital converterfor sampling the quadrature amplitude modulated output constellation andgenerating a calibration symbol selection signal that is coupled to thebit-to-symbol mapper and a gain/phase dither signal that is coupled tothe gain/phase adjustment circuit that randomly dithers either the gainor the phase of the selected quadraphase power element.
 2. In amodulated array transmitter comprising (a) a plurality of quadraphasepower elements that each comprise (1) a QPSK modulator having datainputs for receiving digital input signals, an on/off keying input forreceiving an on/off keying bit input signal that selectively keys themodulator on and off, and a local oscillator input for receiving areference input signal, for modulating the reference input signal inaccordance with the digital input signals to output a modulated RFexcitation signal. (2) a gain/phase adjustment circuit coupled to theQPSK modulator, and (3) a power amplifier coupled to the gain/phaseadjustment circuit for receiving the modulated RF excitation signal, foroutputting an amplified quadrature amplitude modulated output signal,and wherein selected pairs of power amplifiers that are driven withopposing phases to form certain symbols are keyed off by means of therespective on/off keying bit input signal coupled to the respective QPSKmodulator, (b) a power combiner for combining the amplified quadratureamplitude modulated output signals derived from the plurality ofquadraphase power elements to output a quadrature amplitude modulatedoutput constellation containing a plurality of symbols, a method forautocalibrating the modulated array transmitter comprising the steps of:selecting a pair of quadraphase power elements, one of which isadjusted, the other of which is held constant; making a random decisionto dither either the gain or the phase of the selected quadraphase powerelement; if the previous gain or phase dither increases the errormagnitude, reversing the sign of the dither; adjusting the gain or phaseof the selected quadraphase power element y the small dither amount;transmitting a calibration symbol, wherein the two phase-opposed outputsignals of the quadraphase power elements are designed to cancel eachother; measuring the error magnitude; transmitting a predeterminednumber of data symbols; repeating the processing steps starting at thedecision step, until the error magnitude is reduced below a thresholdvalue; selecting a new quadraphase power element, and repeating theabove steps, using the previously selected quadraphase power element asa reference, starting at the decision step.